The comfort of the occupants in buildings depends on the temperature and the humidity of occupied spaces. The temperature in the occupied space can be affected by factors such as the outdoor air conditions, a change in the number of occupants in the space, or devices in the space that are either producing or removing heat. Similarly, the humidity in the occupied space can be affected by outdoor air conditions, the accumulation of water vapor in the space, or processes that deplete water from the air or exhaust water vapor into the air. In the field of air-conditioning, heat sources that cause the temperature to change are typically called sensible loads, while the sources and sinks of water vapor that cause the humidity to change are called latent loads.
Modern air-conditioning systems are designed to compensate for variations in the temperature and the humidity of the occupied space. When conditions arise in which the temperature and/or humidity are higher than desired for comfort, heat pumps operating in cooling mode can provide both cooling and dehumidification. When conditions arise in which the temperature and/or humidity are lower than desired for comfort, heat pumps operating in heating mode can provide heating. Additional humidification can be accomplished by a humidifier.
The two modes in which the heat pump operates, i.e., heating and cooling modes, are similar. The main difference is that the direction in which the refrigerant flows in the cooling mode is opposite to the direction in which the refrigerant circulates in the heating mode. Because of the similarities between the modes, many of the results that apply to one mode also apply to the other mode. This description focuses on the operation of a system providing a net cooling effect. However, it should be understood that analogous results can also be applied to a system providing a net heating effect.
Conventional vapor-compression air-conditioning systems are usually designed to control the temperature in an occupied space, rather than the humidity. When the temperature is low and the humidity is high, such air-conditioning systems do not operate because the temperature is within an acceptable range. While high humidities often accompany high temperatures, this is not always the case. In some climates, summer air temperatures may not be especially high, but people may still feel uncomfortable because of the high humidity. For example, a rainy summer night with temperatures in the range of 20° C. to 22° C. can have a humidity ratio above 6.8 g water/g dry air (dewpoint above 20° C.). Because the sun has set and the air temperature is moderate, the sensible cooling load on a house can be almost zero. If a conventional vapor-compression-based air-conditioner for the house does not operate, the absolute indoor humidity will be equal to or exceed that of outdoors. For a 24° C. indoor temperature, the relative humidity is at least 80%, which is a level that is uncomfortable, and exceeds the 70% threshold at which mold and mildew proliferate.
Thermal comfort can be improved by regulating the humidity in a space. Industrial and commercial processes, e.g., baking or semiconductor fabrication, also often require precise control of room air humidity to reliably produce high-quality products. Building maintenance concerns also justify humidity control, as building structures that are subject to high humidity conditions are prone to damage by mildew and mold.
Systems incorporating vapor-compression air-conditioning equipment can be used to dehumidify spaces, but these systems are inherently quite inefficient in their use of energy. Such systems generally have to cool the air down below the dewpoint in order to achieve the desired absolute humidity of the process air, and then heaters must be used to reheat the overcooled air to achieve the desired temperature of the process air. This process of first dehumidifying and then reheating the process air consumes a great deal of energy.
Some air-conditioning systems control space humidity using desiccants. Desiccants are materials that can remove water vapor from air by the process of either absorption or adsorption. Frequently used desiccants include silica gel packets commonly found in packing materials. Desiccants can be in liquid or solid: solid desiccants embed the desiccant in a matrix or on a substrate over which humid air flows, while liquid desiccants often includes aqueous solutions of hygroscopic salts, such as lithium chloride (LiCl) or lithium bromide (LiBr), of varying concentration. Systems that exchange water vapor from the air with the desiccant substrate typically include a dehumidifier and the regenerator components.
As the desiccant in a dehumidifier accumulates water, the ability of the desiccant to continue removing water from the air decreases, rendering the desiccant less effective. The effectiveness of the desiccant can be renewed by moving the desiccant to a component located in a separate air stream, i.e., the regenerator, that evaporates the water of the desiccant via the application of heat, and vents the water vapor to the outside environment.
A fundamental problem of systems using solid desiccant is that the dried air exiting the dehumidification component is warmer than the input moist air. This additional heat must also be removed from the air stream by the air-conditioning system, reducing the energy efficiency of the overall air-conditioning process. In contrast, a system with liquid desiccant does not generally exhibit this pronounced behavior, and can be used to simultaneously cool and dehumidify the air. Systems using liquid desiccants rely on the physical process of absorption. The heat and mass exchangers effecting the process of dehumidification are called absorbers.
For example, U.S. Pat. Nos. 6,546,746, 4,984,434, 6,684,649, 8,047,511, and 8,268,060 describe examples of liquid desiccant-based air conditioning systems with a few variations, such as the means of regeneration or the types of materials used in some of the components. U.S. Pat. No. 7,966,841 describes an example a particular kind of absorber.
Different spaces in a building often have very different heating and cooling needs. For example, a system, described in U.S. Pat. No. 8,171,746, provides separate latent and sensible cooling in one of the conditioned spaces. The terminal units in the occupied spaces are able to either perform heating or cooling, and either humidification or dehumidification, in each space, independently of the requirements of other spaces in the same building.
While various architectures for a space conditioning system can provide the desired operating conditions, the constraints placed on the system operation by the architecture may cause many of these systems to operate inefficiently. It is therefore desired to provide a space conditioning system that can independently compensate for the latent and sensible loads in a multiplicity of spaces, and can be operated in a manner that optimizes the energy efficiency of the system.